In a Long Term Evolution-Advanced (LTE-A) system, a Coordinated-Multiple-Point (CoMP) technique, which relates to coordination among a plurality of Transmission Points (TPs) geographically separated from each other, is adopted so as to reduce adjacent-cell interference for a cell-edge User Equipment (UE) and improve the user experience. Usually, the plurality of TPs refers to base stations in different cells, or a base station in one cell together with a plurality of Remote Radio Heads (RRHs) controlled thereby. The CoMP technique may include a downlink coordinated-transmission technique and an uplink joint reception technique. There mainly exist two schemes for the downlink coordinated-transmission, i.e., Coordinated Scheduling/Coordinated Beamforming (CS/CB) and Joint Processing (JP). In the CS/CB transmission scheme, one of the plurality of TPs transmits a useful signal to the UE, while the other TPs are scheduled or beamformed jointly so as to reduce the interference on the UE as possible. The JP scheme may further include Joint Transmission (JT) and Dynamic Point Selection (DPS). In the JT scheme, the plurality of TPs transmits a useful signal to an identical UE simultaneously, so as to enhance the signal received by the UE. In the DPS scheme, the TPs for the UE are dynamically switched, so as to always select an optimum TP for the UE from the TPs, thereby to transmit the useful signal to the UE. These CoMP schemes may be used in combination with each other, or in combination with a Dynamic Point Blanking (DPB) scheme, so as to dynamically configure some TPs on some time-frequency resources as not to transmit any signal.
In order to achieve the CoMP transmission for one UE, a network configures a measurement set containing at least one base station for the UE. For each base station in the measurement set, the network configures one or more Channel State Information (CSI) processes for the UE, and the UE reports the CSI in accordance with the configurations for each CSI process.
The network performs the CoMP scheduling in accordance with the CSI reported by the UE, and during the scheduling, it is required to exchange a large quantity of information and data among the TPs. In an LTE-LTE-A system, the interaction of the information and data among the TPs is achieved by Backhaul. An information transmission rate and a transmission delay for Backhaul depend on properties of a physical link for Backhaul and a protocol stack delay. When network nodes are connected to each other via a physical link with large capacity, e.g., directly via an optical fiber, the information transmission rate for Backhaul is large (e.g., at a Gbps level). When the network nodes are connected to each other via a physical link with small capacity, e.g., via a wireless transmission link, the information transmission rate for Backhaul is small (e.g., 1 Mbps or less). The transmission delay for the interaction of information over Backhaul is mainly caused by a physical link transmission delay and the protocol stack delay. Different Backhaul links have remarkably different transmission delays, and usually a non-ideal Backhaul transmission delay may be 10 ms or more. In an actual network, there may exist a plurality of physical connection modes for Backhaul.
The downlink CoMP relies on the CSI transmitted from the UE to the TPs. The CSI includes Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI) and so on. The UE measures a channel from a base station to the UE using downlink reference signals from the respective base stations, and returns the measured CSI via a Serving Cell. A RRH or base station to which the Serving Cell of the UE belongs receives the CSI, and then performs coordinated-scheduling and/or coordinated-precoding with a base station or RRH of a coordination cell, so as to achieve the CoMP. One method for performing coordinated-scheduling and/or coordinated-precoding between the cells is central coordinated-scheduling.
Ideal Backhaul is taken into consideration in the research of CoMP as described in LTE-A Release 11. In the case of the ideal Backhaul, taking the CoMP among the base stations in different cells as an example, the central scheduling mainly includes the following steps.
a. Each coordination base station transmits the latest CSI measured by all the UEs that are connected to the base station to a Central Coordination Node (CCN).
b. The CCN collectively schedules time-domain and frequency-domain resources for the UEs of all the coordination base stations, and precodes the UEs for which the time-domain and frequency-domain resources have been scheduled.
c. The CCN transmits the scheduling and precoding results of the time-frequency resources for each UE to the relevant base stations.
d. The relevant base stations transmit the data to the UE in accordance with the received scheduling and precoding results of the time-domain and frequency-domain resources.
An interface between the CCN and the coordination base station is an X2 interface, so the interaction of the information and data therebetween is achieved via Backhaul.
In the case of the non-ideal Backhaul, when the CoMP central scheduling is performed as mentioned above, the scheduling delay (the time desired from respective coordination base states transmitting CSI to the CCN to they receiving the scheduling and precoding results from the CCN) is at least twice of the Backhaul transmission delay from the CCN to the base station. In addition, when the Backhaul transmission delays from the CCN to the respective coordination base stations are different from each other, the scheduling delay for the coordination base station is twice of the maximum Backhaul transmission delay so as to apply the scheduling result in the respective coordination base stations simultaneously. For a typical Backhaul transmission delay (10 ms), the scheduling delay for the above CoMP central scheduling procedure is substantially 20 ms in the case of the non-ideal Backhaul. It means that, respective coordination base stations perform the CoMP transmission in accordance with the CSI reported by the UE 20 ms ago. The CoMP transmission performance will be seriously affected by the out-of-date CSI.